That should work bar a few newborns that may fit through depending upon the gauge as crabby states. You will need to keep cleaning the area where the slits are as they will get blocked otherwise. That should be easy though, just get a new cheap toothbrush!
Probably not the BacterAE. Cyanobacteria is usually caused by a combination of low flow, excess light, and some sort of nutrient imbalance. I believe phosphate is quite tied into it (too high or too low can cause a bunch of different algaes), and of course high levels of nitrate (waste) will lead to algae outbreaks. Chances are you'll get some more showing up, so maybe drop light levels and test nitrate?